285/80R24.5 vs 305/75R24.5
Going from one to the other changes the overall height by +0.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/80R24.5 | 305/75R24.5 | Change |
|---|---|---|---|
| Overall height | 42.5" | 42.5" | +0.1 % |
| Sidewall | 8.98" | 9.01" | +0.3 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 133.4" | 133.6" | +0.1 % |
| Revs per mile | 475 | 474 | -0.1 % |
| Axle height | 21.23" | 21.26" | +0.8 mm |
| Approved rim width | 8–10.5" | 8.5–11" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 285/80R24.5. Fit 305/75R24.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | +0.0 |
| 50 mph | 50.1 mph | +0.1 |
| 70 mph | 70.1 mph | +0.1 |
| 90 mph | 90.1 mph | +0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,014 on the road, so it under-counts by 14. At a true 70 mph the first turns 554 times a minute, the second 553.